Magnetic random access memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. MRAM differs from volatile random access memory (RAM) in several respects. Because MRAM is non-volatile, MRAM can maintain memory content when the memory device is not powered. Though non-volatile RAM is typically slower than volatile RAM, MRAM has read and write response times that are comparable to that of volatile RAM. Unlike typical RAM technologies that store data as electric charge, MRAM data is stored by magnetoresistive elements. Generally, the magnetoresistive elements are made from two magnetic layers, each of which holds a magnetization. The two magnetic layers are separated from one another by an insulating barrier layer, or a conductive non-magnetic layer such as Cu. When two magnetic layers are separated by a non-magnetic layer that is an insulator, the resulting magnetoresistive devices is called a “magnetic tunnel junction” (“MTJ”). The magnetization of one of the magnetic layers (e.g., the “pinned layer” or “fixed layer”) is fixed in its magnetic orientation, and the magnetization of the other layer (e.g., the “free layer”) can be changed by an external magnetic field generated by a programming current or spin-polarized current through spin transfer torque (STT) phenomenon. Thus, the magnetic field of the programming current or spin-polarized current can cause the magnetic orientations of the two magnetic layers to be either parallel, giving a lower electrical resistance across the layers (logic 0), or antiparallel, giving a higher electrical resistance across the layers (logic 1). The switch in the magnetic orientation of the free layer and the resulting high or low resistance states across the magnetic layers thus enables programming of the typical MRAM cell. However, the high current requirement for the STT induced switching of the MTJ limits the energy-efficiency as well as the switching speed.
In an attempt to find alternate and more energy-efficient switching mechanism for MTJs, spin orbit torque (SOT) phenomenon is considered as a promising way to achieve highly energy-efficient and faster switching of MTJs. SOT switching involves passing a current through a material exhibiting high spin-orbit-coupling (for example, heavy metals like Pt, Ta etc.). Due to the spin-orbit-coupling, the electrical current passing through the heavy metals splits into two spin polarized currents called the up-spin current and the down-spin current. An MTJ formed on top of the heavy metal experiences current induced torque due to such spin polarized currents, which can switch the state of the MTJ. The energy efficiency of the SOT mechanism results from the fact that the spin polarization efficiency of the SOT phenomenon can be much higher as compared to the STT mechanism.
However, such SOT MRAM devices are three- or four-terminal structures as opposed to the two-terminal STT MRAM device. The three or four terminal device structure necessitates use of multiple transistor switches for proper isolation of each bit-cell thereby leading to high area overhead and hence low memory density. Consequently, this also increases the overall energy consumption and also the cost of manufacturing as fewer cells can be patterned on a single silicon wafer.
Accordingly, it would be desirable to provide integrated circuits and methods for fabricating integrated circuits with MRAM structures that exhibit low switching energy by combination of the SOT and the STT phenomena while reducing the number of transistors required per bit-cell resulting in high memory density along with energy efficient switching. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.